1. Field of the Invention
This invention relates to computer systems and, more particularly, to methods and apparatus for controlling writing operations in computer systems in a manner that the data transferred carries information concerning whether a write operation is to occur.
2. History of the Prior Art
In a typical computer system of the prior art, a central processing unit is connected to system memory by an address bus, a data bus, and various control lines. In order to accomplish a write operation, the central processing unit places an address on the address bus indicating the position to which data is to be written, places the data to be written on the data bus, and places control signals on the control lines indicating that a write operation is to occur. The system memory senses the address, determines that the address is one contained within its memory, and responds to the command by writing the data to the addressed position and signalling the central processing unit that the operation is complete.
This form of write operation is carried on throughout the typical computer system for all types of operations. When a write operation deals with a window various complications occur. For example, it is relatively easy to deal with rectangular windows which are to be presented on computer output displays, but it is very much more difficult to deal with other shapes of windows, especially with arbitrarily shaped windows. A central processing unit is capable of forming windows of arbitrary shapes and placing and manipulating data within the areas encompassed by such arbitrarily-shaped windows. A central processing unit is endowed with a great deal of power that may be easily manipulated under program control which allows it to accomplish such operations. However, many other devices which have direct memory access are not so capable and, consequently, are only able to deal with rectangular area. For example, devices called "blitters" used for moving blocks of information to relieve the central processing unit of repetitive graphics operations usually move only rectangular areas. Since such devices are constantly utilized, they become the limiting factor in what can be accomplished by the computer.